Induction heating has been used industrially for a number of years primarily to heat treat and to melt metals. Frequencies used range from AC power line frequencies of 50 or 60 Hz all the way up to about 30 MHz. It is well-known that an electrically conductive material may be inductively heated by placing the material in an alternating magnetic field. Such a material, when inductively heated, is known as a "susceptor". As the magnetic field intensity changes, eddy currents are induced in the susceptor material. Because of resistive losses in the susceptor material these eddy currents heat the material. The frequency at which the magnetic field alternates, the physical size and shape of the susceptor material, its electrical resistance, and in the case of magnetic materials, hysteresis losses of the material, affect the manner in which the susceptor material is heated. The manner in which the magnetic field interacts with a susceptor material is well understood. A conduction path must exist in the susceptor material in a plane approximately normal to the direction of the magnetic field for efficient inductive heating to take place.
A common arrangement for induction heating of a workpiece is to surround the workpiece with a long circuit solenoid-type coil. As current flows through this coil a magnetic field is created. The magnetic field is substantially uniform within the coil and is directed substantially parallel to the axis of the coil. If the coil is energized by an alternating current source, eddy currents are induced in the workpiece. In the situation where a solid workpiece is induction heated, the eddy currents at the surface of the workpiece reduce the intensity of the magnetic field within the interior of the workpiece, thus creating a gradient in the field intensity. This results in weaker eddy currents flowing within the interior of the workpiece than at the surface. The magnitude of this gradient is frequency dependent and also dependent upon the resistivity of the material being heated. At higher frequencies the surface of a solid material is heated much more than its interior region. This phenomenon is also known as the "skin effect". At lower frequencies the workpiece is heated in a more uniform manner. It is common practice to describe a parameter known as the "reference depth" to describe an "equivalent sleeve" which characterizes the so-called "skin effect" in solid objects.